Cheapest by far is 22LR but it tops my "most boring thing ever invented" list. I really can't understand the fun in it. I've shot plenty of 22s. Rifles, handguns, bolt-action, semi-auto, even pump action. 22LR is spelled "b-o-r-i-n-g" in my world.
I find handguns to generally be more fun than rifles but it somewhat depends on what I'm doing. "Blasting away" with handguns is loads of fun. As long as the BOOM! is big-ish it doesn't much matter what cartridge. 9mm, 357sig, 10mm, 45acp... Revolvers in 357, 38, 44mag or spl,....don't matter, they're all fun. Performance wise, I prefer 10mm but if I'm blowing up jugs (or frogs
) any of them will do.
Rifles, I like long(ish) distances and small targets/groups. I don't like recoil so I top out at 7mm-08. 204 Ruger and 22-250 are tons of fun.